Communication systems are known to include a communication resource controller, a plurality of communication units, and communication resources. In such communication systems, the communication units communicate with each other via the communication resource, where a communication resource may be an RF channel, a pair of RF channels, a TDM slot, or any medium for carrying RF signals. To initiate a communication, a communication unit transmits a communication request to the communication resource controller via a communication resource dedicated for control information (control channel). The communication request includes a request for a particular service, such as allocation of a communication resource and identity of a target communication unit, or units. For example, the request may be for a group call which identifies all the communication units in the same group as the initiating communication unit.
Upon receiving this request, the communication resource controller determines whether the communication unit is authorized to access the system and, if so, grants the request. Having granted the request, the communication resource controller allocates a communication resource to the initiating communication unit and the target communication units, and then transmits a communication resource allocation message on the control channel. All communication units within the coverage area of the control channel antenna receives the allocation message. However, only the communication units identified in the message, i.e., the initiating and target communication units, will access the allocated communication resource and subsequently partake in the communication.
In addition to allowing communication units to communicate, many communication systems provide dispatch service. Dispatch service allows an operator to communicate with one or many communication groups and provides a central point of communication. For example, if the communication system is used by a police force, the operator may be directly coupled to the 911 service provided in that area, and upon receiving a 911 call, can communicate with a police officer in the field who, in turn, can respond to the 911 call. If the 911 call is reporting a fire, the operator may also be in communication with the fire department, which is a separate communication group than the police force. However, this service does not provide the units responding to the call with any information as to when other units will be arriving at the scene. In many emergency situations, the estimated time of arrival of assistance is crucial. For example, if a police officer is responding to an armed robbery his or her response will be different depending on how soon back-up will be on the scene. If back-up is 15 minutes away, the officer may have to individually handle the situation, where, if the back-up is a minute away, the office can wait until the back-up arrives.
Therefore, a need exists for a method that provides estimated time of arrivals of communication units.